This invention pertains to an improvement in methods for fusing bristles into a brush, and more specifically, to a configuration for a handle for use in fusion processes.
Most brushware, especially toothbrushes, are produced via a two-part technique. The handles are produced by injection molding, following which the bristles are inserted into the handle. The most common technique for inserting the bristles into the brushes is stapling. The bristles are folded around a metal staple which is pushed into a pre-molded hole in the brush. The staple cuts into the plastic at the periphery of the hole, and the plastic retains both it and the bristles. However, it does not require great force to remove the bristles from the handle. Vigorous brushing can easily cause the bristles to be removed from the handle, leading to shedding or even release of the metal staple inside of the mouth.
Techniques wherein the bristles are fused with the handle can be used to produce brushes from which the bristles are not so readily removed. Either the bristles or the brush head, or both, are heated, and the bristles are inserted into holes in the handle where they are retained by the cooling plastic. Exemplary techniques for brush production by fusion include those described in U.S. Pat. No. 4,988,146, which describes a fusion process wherein the ends of bristle bundles are thermally fused, shortening and locally thickening the bundles to form a fuse-ball, or fuse. The fused bristles are inserted into holes in a brush handle which have a smaller cross-section than the fuse-ball. Either the fuse-ball or the wall of the hole may be heated to allow the fuse-ball to conform to the inside of the hole, or the bristles may be inserted into the hole immediately following fusion, before the fuse-ball is allowed to cool.
U.S. Pat. No. 5,224,763 discloses a fusion process in which holes are formed in the handles during the injection molding process. A collar of excess plastic disposed about the hole is swaged around the fused end of the bristles when it is inserted into the hole.
U.S. Pat. No. 5,622,411 discloses a fusion process wherein it is assumed that the fused bristles will displace a finite amount of material when they are inserted into holes in the handle head. The displaced material is compressed to form a planar surface in the head of the brush.
In each of these techniques, the inventor has sought a method of inserting bristles into pre-cored holes in the handle. However, it is not necessary to use a handle with prefabricated holes; the holes can be formed immediately prior to the insertion of the bristles. In a process called xe2x80x9chedgehogging,xe2x80x9d the handle is heated, and a set of short spikes mounted on a heated plate is pushed into the head of the handle to form holes to receive the bristles. U.S. Pat. No. 4,637,660 describes an exemplary hedgehogging process wherein, as the holes are formed in the handle, the displaced material is organized into a small bead surrounding the newly-formed hole. As in the ""146 patent, the bristle ends are fused before they are inserted into the hedgehogged holes. Material from the small bead flows around the fuse after it is fitted into the hole, enclosing the bristles in the brush head.
In most of these methods, a significant portion of the brush must be heated before the bristles are inserted into the hole whether it is pre-cored or hedgehogged. The only exception is the ""146 patent, where the heated fuses may be inserted into the hole without heating the handle. In this case, the fuse expands against the walls of the hole, thermoforming the fuse and exerting hydrostatic pressure on the walls of the hole. In either case, excess post-molding processing of the handle will weaken the head and may cause it to warp. In addition, the fracture toughness of the head may be reduced. Many users bang their toothbrushes against the side of the sink to remove excess water after they are done brushing their teeth. This action is more likely to break a warped or pre-stressed brush head.
In one aspect, the invention is a bristle carrier for a brush which includes a bristle receiving portion, at least one pre-molded hole disposed in the bristle receiving portion, and a projection disposed in a bottom of the pre-molded hole. The hole is configured to receive a bristle tuft. A side surface of the protrusion may be perpendicular to the bottom of the hole, parallel to a wall of the hole, both, or neither. An upper surface of the protrusion may also exhibit a 3-dimensional contour. In addition, the wall of the hole need not be perpendicular to the bottom of the hole; it may be rounded or flat. A hole may include a plurality of protrusions; in a brush having a plurality of holes, the holes need not all have the same shape, and the shapes of the protrusions disposed in the holes may also vary. In addition, a wall of the hole need not be perpendicular to a surface of the bristle receiving portion that contains the opening of the hole.
In another aspect, the invention is a method of producing a brush. The method comprises forming a bristle carrier having at least one hole with a protrusion projecting from its bottom, heating the protrusion and a portion of the wall of the hole, and inserting a sheaf of bristles into the hole. When the protrusion is heated, material from it flows about the bristles, retaining them in the hole. The method may further comprise fusing an end of the sheaf of bristles to form a fuse-ball. This fuse may have a greater diameter than the diameter of the sheaf, and the fusion may be performed thermally or chemically. If the fusion is performed thermally, the sheaf may be inserted in the hole while the fuse-ball is still warm. A portion of the wall of the hole may be pressed around the fuse. Furthermore, the bristle carrier may be formed by either injection or compression molding.